The present invention relates to a method for producing a corn milling residue carboxymethyl ether salt.
Starch milling residue (coarse grains on sieve) available as a byproduct of starch industry contains, in addition to the major components starch and cellulose, large proportions of crude protein, ashes and other impurities (composition of a typical starch milling residue: starch 54.4%, crude fiber 18.6%, ashes 8.7%, crude protein 1.8%, etc.; cf. Jiro Nikuni (ed): Denpun Kagaku Handbook (Handbook of Starch Science), Asakura Shoten, 1977). For this reason, there is not known a process for converting such a milling residue to a carboxymethyl ether salt on a commercial scale and the only use for such byproduct that is known today is as an inexpensive feedstuff.
On the other hand, the technology of carboxymethylating each of starch and cellulose, both of which are major components of the milling residue as aforesaid, is well known. For example, it is known to react each of them with chloroacetic acid or a salt or derivative (e.g. ester) thereof in the presence of alkali in a reaction medium such as water, a mixture of water and a hydrophilic organic solvent such as 2-propanol or a mixture of ethanol, benzene and water.
However, if such a conventional process for carboxymethylation of starch is applied to a corn milling residue, the aqueous slurry will not be sufficiently homogenous because the contaminant cellulose is hard to react. On the other hand, if the conventional carboxymethylation process for cellulose is applied to the corn milling residue, the concomitant starch is gelatinized in the course of reaction so that no satisfactory result can be obtained, either.
In addition, since the conventional process employing organic solvents (the so-called organic solvent process) involves the use of different solvents for reaction and purification, the reclaiming of used solvents imposes a serious limitation on the operation. For example, when 2-propanol is used for the reaction solvent, the recovery of the solvent after completion of the reaction is hampered by gelatinization of the system. If it is not recovered, however, 2-propanol finds its way into the purification solvent (aqueous methanol) and fractional distillation becomes essential for separation of the two solvents.
In the aqueous solvent process employing water, the reaction must be conducted at low temperature for a protracted time in order to avoid gelatinization which is inevitable at high temperature, so that the productivity is sacrificed of necessity.
A process using methanol as the solvent is also known (Japanese Patent Publication No. 27-5295) but it is not effective for carboxymethylation of a corn milling residue and, hence, does not yield a water-soluble reaction product.
Aside from the above-mentioned processes, it has been proposed to carboxymethylate starch in slightly wet condition (Japanese Patent Publication No. 59-41642) but the degree of substitution of the product is in the neighborhood of 0.3. Furthermore, application of this process to a corn milling residue does not yield a homogenous solution.